Biofouling is the accumulation of microorganisms, plants, algae, or animals on wetted surfaces. Marine biofouling begins with the adhesion of a conditioning film of proteins and polysaccharides within minutes of submerging an artificial surface in seawater. Within a day, bacteria and single cell diatoms can attach to the conditioning film, initiating the formation of a biofilm. Within weeks, colonizers, such as algal spores, barnacle cyprids, and marine fungi, become attached to the biofilm to form a thick coating. Such biofouling increases the weight, hydrodynamic friction, and corrosion of shipping vessels and other marine systems (see, e.g., Majumdar P et al., “Combinatorial materials research applied to the development of new surface coatings XV: an investigation of polysiloxane anti-fouling/fouling-release coatings containing tethered quaternary ammonium salt groups,” ACS Comb. Sci. 2011; 13:298-309.
For example, marine hydrokinetic (MHK) technologies harness the energy of water as it moves and are vital in providing clean and sustainable energy. Biofouling presents an ongoing problem for MHKs as water organisms cling to their surfaces, resulting in a decrease in operation efficiency. Development of effective antifouling coatings to facilitate easy removal of organisms, or prevent adhesion, would ensure MHKs continue to operate at optimal efficiency over time.
Historically, antifouling coatings have been used to prevent or delay the biofouling accumulation. Traditional commercial coatings have included biocides comprising chemical substances incorporated into an antifouling surface coating that deter or kill the microorganisms responsible for formation of the biofilm. The most common biocides are tributyltin (TBT), leachable cuprous oxide, and self-polishing copolymers with alkyl silane functional monomers. However, these biocides are persistent toxins to larger aquatic organisms and are being phased out. In addition, low-toxicity fouling release and antifoulants, such as polysiloxane and quaternary ammonium salts have also been investigated. The hydrophobic polysiloxanes prevent adhesion of larger microorganisms and enable the release of fouling organisms at high speed, but have problems with mechanical strength and long term stability.
Many of these coatings rely on ammonium salts that are amphoteric surfactants (see, e.g., McBain A J et al., “Effects of quaternary-ammonium-based formulations on bacterial community dynamics and antimicrobial susceptibility,” Appl. Environ. Microbiol. 2004; 70(6):3449-59). The antimicrobial and antibacterial mode of action of the quaternary ammonium salts has been described for ammonium salts that contain a long chain aliphatic moiety which can penetrate a cell wall leading to lysis (Hugo W B, “The mode of action of antibacterial agents,” J. Appl. Bacteriol. 1967; 30(1):17-50). Ammonium-based coatings have been used for greater than 50 years due in part to their broad spectrum antimicrobial activity (see, e.g., Pant R R et al., “Synthesis and biocidal efficacy of self-spreading polydimethylsiloxane oligomers possessing oxyethylene-functionalized quaternary ammoniums,” J. Appl. Polym. Sci. 2009; 113:2397-403). Benzalkonium chlorides have been increasingly used as additives in disinfectant cleaning formulations.
Current estimates indicate that the total US anti-microbial market will reach close to $1 billion by the year 2015 (see, e.g., “Anti-microbial Coatings: A US Market Report” (Global Industry Analysts, Inc., 2011). As the market size increases, there is a real need for the development of novel high performance antimicrobial and antibacterial coatings. Furthermore, some organisms have become resistant to the ammonium-based antimicrobials and antibacterials, necessitating the need for alternative coating methodologies (Heir E et al., “Resistance to quaternary ammonium compounds in Staphylococcus spp. isolated from the food industry and nucleotide sequence of the resistance plasmid pST827,” J. Appl. Bacteriol. 1995 August; 79(2):149-56).
One of the largest problems of current coatings is that fact that they are not long lived because additional microorganisms eventually colonize the remains of the cells that were lysed initially by the antifouling coatings. Therefore, a need remains for an alternative type of antifouling coating capable of facilitating the removal of the dead organisms and accumulated biofilm.